The present invention relates generally to surge suppression systems. More specifically, the present invention relates to surge suppression systems provided at the electrical service entrance. Even more specifically, the present invention relates to a simple, highly effective and reliable meter extender surge suppression system.
Modern life is defined by our dependence on electrical appliances and equipment. The microwave, the television, the VCR and the computer are just some of the standard goods in a contemporary home that rely on electricity for power. Likewise, computers, medical equipment, and communications equipment can be found in a variety of business environments. With such a substantial investment of resources, these products merit protection.
One potential hazard to these devices is electric surges delivered to the home or business over the electric utility lines or ground conductors. Over-voltage surges may be caused, for example, by lightning strikes or power frequency over-voltages. In an attempt to minimize or eliminate damage that would otherwise result from these surges, surge suppression apparatus and systems were introduced. A surge arrester is commonly connected in parallel with a comparatively expensive piece of electrical equipment to shunt over-voltage surges to ground, thereby protecting the equipment and circuits from damage or destruction. Placing surge arresters at the electrical entrance to the house or business is one way to protect against over-voltage surges. A convenient means to house and electrically connect such arresters is in a meter extender or meter adapter. A meter adapter is a device that is typically installed between a conventional electric watt-hour meter and the meter socket of a customer's meter box. The watt-hour meter records the power consumed by the customer's loads that are connected downstream of the meter. The meter box itself is typically installed immediately adjacent to and upstream of the customer's distribution panel, breaker box, or fuse box. The meter adapter plugs into the meter box and acts as a socket to receive the meter. Prior art meter extenders typically have included a housing and a number of voltage-dependent, non-linear resistive elements retained on or within the housing, as well as a pair of electrical terminals for connecting the surge arrester between line and ground. The voltage-dependent, non-linear resistive elements employed are typically, but not restricted to, metal oxide varistor (MOV) elements.
A varistor is a voltage-sensitive resistor. The volt-ampere characteristic is highly non-linear. That is, as the voltage increases across the MOV element, the dynamic resistance decreases and the current flow through the MOV increases rapidly. This phenomenon is due to the material's electronic response at an atomic level and not due to thermal effects (in contrast to thermisters). Nonetheless, heating of the varistor tends to magnify this effect. Due to the grain structure of the varistor at the atomic level, a high break-down voltage tends to reflect a high steady state resistance whereas a low break-down voltage tends to reflect a low steady state resistance.
The varistor elements provide either a high or a low impedance current path between the terminals of the voltage arrester or suppresser, depending on the voltage appearing across the varistor elements themselves. More specifically, within the power system steady state or normal operating voltages, the varistor elements have a relatively high impedance. As the applied voltage increases, gradually or abruptly, their impedance progressively declines until the voltage appearing across the varistors reaches what is known as the element's break-down voltage. At the break-down voltage, impedance dramatically decreases and the varistor elements become highly conductive. In this highly conductive mode, the varistor elements serve to conduct the transient over-voltage induced current to ground. As the transient over-voltage and resulting current dissipate, the varistor elements' impedance once again increases, restoring the arrester and electrical system to its normal, steady-state condition.
Occasionally, the transient condition may cause some degree of damage to one or more of the varistor elements. Such damage may lead to a condition known as "thermal runaway." When the varistor conducts transient currents to ground, heat is generated. Heating of the varistor results in lowering the resistance of the varistor and allows higher currents to flow. These higher currents tend to heat the varistors further, which, in turn, lowers the impedance of the varistor still further and allows still higher currents to flow. Excessive heating of the varistors lowers their resistance to such an extent that they become unable to cut off the flow of transient over-voltage current to ground. Ultimately, the varistors are destroyed and their resistance stays low, even after the over-voltage condition has disappeared.
Another manner in which varistors can be damaged is by arcing caused by flashover. Broadly, flashover is the term that describes when undesired electric arcing occurs between high and low potential points. Flashover may occur during over-voltage conditions. When two or more varistors exist in a surge suppression system, and one has a terminal at a significantly higher voltage than some point on the other varistor, arcing between the high and low potential locations can occur.
Varisters in prior art systems were sometimes placed in parallel to improve performance. So connected, however, the varistors may be susceptible to "cascaded failure." If the two (or more) varistor elements are not perfectly or nearly perfectly matched, one of the varistor elements tends to carry more current during over-voltage conditions than the other. This imbalance between or among varistors results in greater heating of one of the varistor elements, resulting in that varistor conducting a greater amount of current. This thereby leads to the varistor's ultimate destruction and failure due to thermal runaway. Sometimes, a varistor may fail violently, exploding into shrapnel-like fragments. In such an event, the remaining varistor (if not damaged by the failure of the first varistor) is left to handle another over-voltage condition. Often, this is more than the remaining varistor(s) can handle, and another varistor fails.
Certain of these problems are recognized in the prior art, and various inventors have tried to solve them. For example, Allina U.S. Pat. No. 4,931,895 ('895) discloses a meter base extender disposed between a conventional watt-hour meter and the meter socket of an electric utility box or panel. To reduce the chances of thermal run-away, Allina provides a heat sink connected to the grounded side of the varistors to dissipate thermal energy. However, this solution requires the extra cost of a heat sink, and the unit is not fully operational until the meter extender is assembled.
Despite surge suppressers' uncontested value, consumers are reluctant to spend a significant amount of capital for surge arrester or suppression systems since they may never be required. This reluctance often means that available products do not inherently include surge protection. A case in point is found in electronics. Due to the competitive nature of the electronics industry, transient protection circuitry is often overlooked to keep costs down. Nonetheless, multiple levels of protection, such as at the distribution transformer, service entrance, and at the electronic device itself provide the best protection. To complicate matters, the surge protection industry itself is very competitive, so keeping costs down, reliability up, and customer satisfaction as high as possible are particularly important to those in the industry. Surge protectors that achieve these goals are most likely to succeed in the marketplace.
Prior art devices often had significant shortcomings in one or more of these areas. As stated above, the surge arrester disclosed in Allina '895 requires that it be fully assembled in order to operate (Col. 4, lines 27-48). Such an approach necessarily creates alignment and assembly difficulties since it depends on closure of mating parts to make the requisite electrical contacts. Further, testing of the device can only occur after assembly. Still further, it is difficult to determine whether an inoperable Allina device is flawed in manufacture, was broken during assembly or shipping, or broken during installation, so the ultimate flaw causing the breakage may be difficult to pinpoint and solve.
Lindsay U.S. Pat. No. 5,023,747 teaches another meter-based surge suppression system. Once again, the invention must be fully assembled before being operational and therefore has similar limitations to Allina '895. Also, Lindsay depends upon assembly to electrically connect intricate elements of his device, thereby increasing the difficulty of his assembly. Such an approach decreases reliability first because the chances of breakage during assembly are higher than necessary, and second because any improperly connected elements may lead to failure of the device. Further, Lindsay depends on a circuit to emit a sound when his fuses have blown. However, surge suppression systems exist because electrical surges on the line may destroy electronic components. Lindsay's reliance upon electronic components to notify an observer that a fuse has blown could be misplaced if any of the electronic components are destroyed by the surges on the line.
Certain other protection schemes of this general type have contained a neon indicator or LED to inform the user that the system is operational or non-operational. In some such previous designs, one varistor in the system could be destroyed and the indicator light would continue to illuminate, albeit at a lower intensity. However, distinguishing between a dim light and a fully illuminated light can be difficult for the consumer, especially considering that not all lights have equal full intensities and the consumer views the light in various degrees of sunshine and darkness.
Therefore, a need exists for a low-cost, high-reliability surge arrester device. Ideally, this device could be tested before assembly with a minimum of time and resources. Further, the device would be easy to assemble and would resist breakage during assembly and installation. The ideal device could be easily assembled and would include a minimal number of components. Its longevity and reliability should also be higher than that known in the prior art. Lastly, the ideal device would preferably provide a clear, reliable indication to an observer whether varistor elements are operational.